Electron guns used in color picture tubes, such as for use in television, are required to achieve good electron beam spot behavior over the entire screen. This requirement is complicated by the presence of astigmatic yoke fields that are necessary to maintain convergence of three beams over the entire screen. In tubes which feature dynamic astigmatism control, the yoke-induced astigmatism is corrected through modulation of the voltage applied to the electrodes in the electron gun or by magnetic components located on the exterior of the tube neck.
In tubes that do not feature dynamic astigmatism control, a reasonable compromise between performance at the screen center and at locations near the periphery of the screen must be achieved. This compromise is usually made in one of two ways. First, astigmatism can be added to the beam in the electron gun so that the spot at the screen center is vertically underfocused when it is at its best horizontal focus. At the periphery of the screen, this astigmatism then cancels some of the vertical overfocusing caused by the yoke. The second technique is to reduce the vertical beam size in the main focus lens. This second technique tends to reduce the variation of vertical spot size at the screen caused by focus voltage changes, and also reduces the magnitude of the yoke-induced astigmatism. As a result, vertical spots overfocused by the yoke are not degraded to the same extent as those with larger vertical beam sizes in the main focus lens. Both of these techniques, however, improve vertical spot uniformity of the deflected beams at the expense of degrading vertical spot size at the screen center.
Beam astigmatism is typically introduced into the gun through design of the main focus region electrodes. The vertical beam size, entering the main focus region, is generally controlled independently from the horizontal beam size, through the introduction of slots or other shaped apertures or recesses into the beam-forming region or prefocus region of the electron gun. Astigmatism and vertical beam size, and hence vertical spot uniformity, can be adjusted at some particular beam current, through appropriate design of the main focus lens in combination with a beam-forming region slot.
The concept of forming an astigmatic lens in the beam-forming region of an electron gun by the inclusion of a slot in the first electrode grid is disclosed in the following U.S. Patents: U.S. Pat. No. 4,242,613, issued to J. Brambring et al. on Dec. 30, 1980; U.S. Pat. No. 4,25 1,747, issued to G. A. Burdick on Feb. 17, 1981; U.S. Pat. No. 4,272,700, issued to F. K. Collins on Jun. 9, 1981; and U.S. Pat. No. 4,558,243, issued to Bechis et al. on Dec. 10, 1985. Slots in the second electrode grid are disclosed in the following U.S. Patents: U.S. Pat. No. 3,497,763 issued to J. Hasker Feb. 24, 1970; U.S. Pat. No. 3,866,081 issued to J. Hasker et al. on Feb. 11, 1975; and U.S. Pat. No. 4,234,814 issued to H. Y. Chen at al. on Nov. 18, 1980.
The beam ellipticity that can be achieved by slot optics in the beam-forming region is limited by fabrication and assembly constraints. In some guns, slot shaped recesses are placed around each of the three apertures in the G1 electrode. The stamping process used limits the depth and width of the slots to dimensions that produce relatively small degrees of ellipticity (about 1.5:1). Alternative approaches, such as an open crossed slot G1 grid, can achieve the desired ellipticity (&gt;1.7:1), but at the expense of more complicated fabrication and assembly processes. The use of strong slots in the beam-forming region can also result in highly non-uniform beams at high currents, leading to large spots at the screen. Slots in the beam-forming region can reduce vertical beam growth with increasing beam current, when compared to beam-forming regions with round-optics. This reduced vertical beam growth can have a beneficial effect on spot uniformity.
An additional important consideration in electron gun design is how vertical spot uniformity evolves with beam current. Because vertical flare is particularly objectionable at high currents, an increase in astigmatism with beam current to minimize the overfocus flare of high-current deflected spots can be beneficial. Additionally, there is a need to minimize the increase in vertical beam size with increasing current.
Slots located in the prefocus lens region of the gun can produce, for some intermediate beam currents (less than 2000 .mu.A), the desired degree of beam ellipticity required to achieve a horizontal beam size sufficiently large for a given level of horizontal resolution, and at the same time, a vertical beam size sufficiently small to obtain the desired degree of vertical spot uniformity.
The use of slots in the prefocus lens region of a gun is shown in U.S. Pat. No. 4,877,998, issued to Maninger et al. on Oct. 31, 1989. In that patent, the slots are shaped apertures in the G4 electrode. The apertures in the G4 are elongated in the inline direction of the beams, whereby each aperture includes a substantially circular center portion and two oppositely disposed arcuate portions that intersect the circumference of the circular center portion.
The above-mentioned patents provide various contributions to the cathode-ray tube art, but they do not suggest how the concepts disclosed therein can be combined to obtain an electron gun having decidedly improved performance at higher beam currents (e.g., above 2000 .mu.A), without using dynamic astigmatism control.